See error handling
documentation for a detailed explanation of the mechanism of handling
errors, including the common "bad" arguments to distributions
and functions, and how to use Policies
to control it.

But, by default, exceptions will be raised,
for domain errors, pole errors, numeric overflow, and internal evaluation
errors. To avoid the exceptions from getting thrown and instead get an
appropriate value returned, usually a NaN (domain errors pole errors
or internal errors), or infinity (from overflow), you need to change
the policy.

The following example demonstrates the effect of setting the macro BOOST_MATH_DOMAIN_ERROR_POLICY
when an invalid argument is encountered. For the purposes of this example,
we'll pass a negative degrees of freedom parameter to the student's t
distribution.

Since we know that this is a single file program we could just add:

#defineBOOST_MATH_DOMAIN_ERROR_POLICYignore_error

to the top of the source file to change the default policy to one that
simply returns a NaN when a domain error occurs. Alternatively we could
use:

#defineBOOST_MATH_DOMAIN_ERROR_POLICYerrno_on_error

To ensure the ::errno
is set when a domain error occurs as well as returning a NaN.

This is safe provided the program consists of a single translation unit
and we place the define before
any #includes. Note that should we add the define after the includes
then it will have no effect! A warning such as:

Next we'll define the program's main() to call the student's t distribution
with an invalid degrees of freedom parameter, the program is set up to
handle either an exception or a NaN:

intmain(){cout<<"Example error handling using Student's t function. "<<endl;cout<<"BOOST_MATH_DOMAIN_ERROR_POLICY is set to: "<<BOOST_STRINGIZE(BOOST_MATH_DOMAIN_ERROR_POLICY)<<endl;doubledegrees_of_freedom=-1;// A bad argument!
doublet=10;try{errno=0;// Clear/reset.
students_tdist(degrees_of_freedom);// exception is thrown here if enabled.
doublep=cdf(dist,t);// Test for error reported by other means:
if((boost::math::isnan)(p)){cout<<"cdf returned a NaN!"<<endl;if(errno!=0){// So errno has been set.
cout<<"errno is set to: "<<errno<<endl;}}elsecout<<"Probability of Student's t is "<<p<<endl;}catch(conststd::exception&e){std::cout<<"\n""Message from thrown exception was:\n "<<e.what()<<std::endl;}return0;}// int main()

Here's what the program output looks like with a default build (one that
does throw exceptions):

Example error handling using Student's t function.
BOOST_MATH_DOMAIN_ERROR_POLICY is set to: throw_on_error
Message from thrown exception was:
Error in function boost::math::students_t_distribution<double>::students_t_distribution:
Degrees of freedom argument is -1, but must be > 0 !

Alternatively let's build with:

#defineBOOST_MATH_DOMAIN_ERROR_POLICYignore_error

Now the program output is:

Example error handling using Student's t function.
BOOST_MATH_DOMAIN_ERROR_POLICY is set to: ignore_error
cdf returned a NaN!

And finally let's build with:

#defineBOOST_MATH_DOMAIN_ERROR_POLICYerrno_on_error

Which gives the output show errno:

Example error handling using Student's t function.
BOOST_MATH_DOMAIN_ERROR_POLICY is set to: errno_on_error
cdf returned a NaN!
errno is set to: 33

Caution

If throwing of exceptions is enabled (the default) but you do not have try & catch block, then the program
will terminate with an uncaught exception and probably abort.

Therefore to get the benefit of helpful error messages, enabling all exceptions and using try & catch is
recommended for most applications.